• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Cylinder #1 running hot

Gsjj

CB750 Member
Messages
23
Reaction score
0
Points
1
Location
Tampa
Hello all,

I am working on my 83 nighthawk 750 and just got her started after about 5 years of sitting. My biggest problem right now is a very rough idle and the #1 cylinder running very hot. It is seems to be changing color on the first bend out of the cylinder. What would cause excessive heat?

Thanks for your help.
 
Hello all,

I am working on my 83 nighthawk 750 and just got her started after about 5 years of sitting. My biggest problem right now is a very rough idle and the #1 cylinder running very hot. It is seems to be changing color on the first bend out of the cylinder. What would cause excessive heat?

Thanks for your help.

No one has ever had this problem?

I pulled the plugs today and all 4 had a different outcome.

#1 was practically white, running hot

#2 was just right

#3 has visible oil on it and was dark

#4 was the darkest, but no visible oil.


Is it possible that all 4 will need completely different mixture adjustments or is some else wrong?
 
Checked compression? Checked for vacuum leaks? Carbs/jets plugged? More then likely a combination of all of them causing issues.
 
Hello,

I just rebuilt and cleaned carbs thoroughly, they are in great shape. I have to rent a compression tester in the next few days as well as a carb synchronizer. The boots are in good shaped with no cracks, but they seem to not fit correctly. That could be one issue, I will compression test first, get the bike back together, and check for the vacuum leaks.

Will the fuel mixture settings affect whether or not the one cylinder runs hot?

Thanks for the input.
 
Who knows what could have happened with 5 years of sitting. First thing I did was remove the headers and carb/boots and looked down inside the block at the valves. My bike was a barn sitter. So glad I did this inspection because if I tried running the motor, who knows what I was sucking inside and what further damage could have happened.

Look closely

20170502_000848.jpg
 
The #1 cylinder is still running hot after backing out the fuel mixture screw as much as possible. My father, who purchased the bike new in 83, said the left side (cylinder 1 and 2) always ran hot, maybe that is just something to live with? Aside from sitting for 5 years, my father kept excellent care of the bike and it never sat outside nor saw a rainy day. Just lack of maintenance the past few years due to his health.

My problem now is that after spending time on a full carb clean and partial rebuild and driving it 5 miles, I could not get it to idle and drive again. After some thought, I removed the Pet-cock and the filter was just about disintegrated and I am sure whatever rust that was in the tank has now clogged up the jets again!

So, I need a new fuel filter, but it does not seem like I can purchase this separately any longer, only comes with a new pet-cock for $110. Anyone have a link to the filter only?
 
Who knows what could have happened with 5 years of sitting. First thing I did was remove the headers and carb/boots and looked down inside the block at the valves. My bike was a barn sitter. So glad I did this inspection because if I tried running the motor, who knows what I was sucking inside and what further damage could have happened.

Look closely

View attachment 8036

I can't tell what that is
 
The pipe turns blue because it's lean. Carb likely clogged and most who clean them cannot get that clog gone. What was used as the cleaning solvent? If good enough to remove varnish then likely you ruined the air cut valves if the carbs were not broken up into individual ones to remove those diaphragms. That can wreck the idle quality to idle poorly or not even start.

Left side running hot would be a problem, they do NOT do that. You set all 4 idle screws the same and carb has issues if that doesn't work. 2-2 1/2 turns out. Pick one and make them the same. Plugs should all come out looking about the same too.

If the bike has run enough in the past to cook the intake manifold rubbers then removing carbs cracks them to leak and it won't matter if you use PB blaster or other to get them back on, leaks are leaks.

Often a carb rebuild has them running worse than before, it reflects on the skills of the person doing the work. Those carbs are among the b-tchiest on the planet about getting right, there is no almost to it, you have to do the work 100% perfectly and ALL of it ALL the way.
 
Hello,

Thank you for the input. I used the "gunk removal" sold at the auto parts stores. I did not separate the carbs, where are the air cut off valves. I have them off the bike now as I decided to order the new petcock valve as that was trashed either way.
 
Look at the left side carb as you sit on bike, the two screw round cover on the side, they all have it, valve is under it.
 
Update.. Yesterday I received my new pet-cock. I had already removed the carb and did a check on all the jets and maded sure everything was free and clear. Adjusted the fuel mixture screws to spec and then I installed the new pet-cock.

Magic happened! She started right up, idled perfectly at around 1400 and never looked back. It ran completely different. The original repair of a carb rebuild was needed, but to find out the pet-cock also was a major issue was great news. By the way, all cylinders were equal hot, crap from the bad pet-cock must have clogged it up.

Now (as long as long as this holds up) I can focus on other much needed maintenance and adjustments. The front brakes will be first.
 
If the engine is idling that perfectly then it will idle at a lower speed, lower it to around 1100. 1400 is one big reason why things are overheating, the bike was never intended to idle that fast and overheats.
 
To be honest, the RPM needle is very jumpy and an accurate reading can not be read. (Not sure if that can be repaired?) The needle jumps anywhere from 1-1500 RPM, I ran it again last night and I will continue to adjust it. The main point is that is does not stall at lower the RPM and is running great. Thanks for your help.
 
You want the lowest rpm the engine will run smoothly at, too fast and the tensioners cook much faster and what often brings the engines down when they break due to the rubber overcooked.

The tach can be made much better by pumping a SMALL amount of grease into the bottom fitting where the cable goes, the bushing dries up and then it skitters inside the outer housing to make the needle jump. Of course the cable must be free and greased too.
 
Back
Top